Subject: Re: [PATCHv2 bpf-next 1/4] kallsyms: Add kallsyms_lookup_names function

From: Masami Hiramatsu <[email-masked]>
Date: Tue, 26 Apr 2022 19:01:08 +0900

Hi Jiri,

Sorry for replying late.

On Fri, 22 Apr 2022 08:47:13 +0200
Jiri Olsa <[email-masked]> wrote:

> On Tue, Apr 19, 2022 at 10:26:05AM +0200, Jiri Olsa wrote:
> 
> SNIP
> 
> > > > +static int kallsyms_callback(void *data, const char *name,
> > > > +			     struct module *mod, unsigned long addr)
> > > > +{
> > > > +	struct kallsyms_data *args = data;
> > > > +
> > > > +	if (!bsearch(&name, args->syms, args->cnt, sizeof(*args->syms), symbols_cmp))
> > > > +		return 0;
> > > > +
> > > > +	addr = ftrace_location(addr);
> > > > +	if (!addr)
> > > > +		return 0;
> > > 
> > > Ooops, wait. Did you do this last version? I missed this point.
> > > This changes the meanings of the kernel function.
> > 
> > yes, it was there before ;-) and you're right.. so some archs can
> > return different address, I did not realize that
> > 
> > > 
> > > > +
> > > > +	args->addrs[args->found++] = addr;
> > > > +	return args->found == args->cnt ? 1 : 0;
> > > > +}
> > > > +
> > > > +/**
> > > > + * kallsyms_lookup_names - Lookup addresses for array of symbols
> > > 
> > > More correctly "Lookup 'ftraced' addresses for array of sorted symbols", right?
> > > 
> > > I'm not sure, we can call it as a 'kallsyms' API, since this is using
> > > kallsyms but doesn't return symbol address, but ftrace address.
> > > I think this name misleads user to expect returning symbol address.
> > > 
> > > > + *
> > > > + * @syms: array of symbols pointers symbols to resolve, must be
> > > > + * alphabetically sorted
> > > > + * @cnt: number of symbols/addresses in @syms/@addrs arrays
> > > > + * @addrs: array for storing resulting addresses
> > > > + *
> > > > + * This function looks up addresses for array of symbols provided in
> > > > + * @syms array (must be alphabetically sorted) and stores them in
> > > > + * @addrs array, which needs to be big enough to store at least @cnt
> > > > + * addresses.
> > > 
> > > Hmm, sorry I changed my mind. I rather like to expose kallsyms_on_each_symbol()
> > > and provide this API from fprobe or ftrace, because this returns ftrace address
> > > and thus this is only used from fprobe.
> > 
> > ok, so how about:
> > 
> >   int ftrace_lookup_symbols(const char **sorted_syms, size_t cnt, unsigned long *addrs);
> 
> quick question.. is it ok if it stays in kalsyms.c object?

I think if this is for the ftrace API, I think it should be in the ftrace.c, and
it can remove unneeded #ifdefs in C code.

> 
> so we don't need to expose kallsyms_on_each_symbol,
> and it stays in 'kalsyms' place

We don't need to expose it to modules, but just make it into a global scope.
I don't think that doesn't cause a problem.

Thank you,

> 
> jirka
> 
> 
> 
> diff --git a/include/linux/kallsyms.h b/include/linux/kallsyms.h
> index ce1bd2fbf23e..177e0b13c8c5 100644
> --- a/include/linux/kallsyms.h
> +++ b/include/linux/kallsyms.h
> @@ -72,6 +72,7 @@ int kallsyms_on_each_symbol(int (*fn)(void *, const char *, struct module *,
>  #ifdef CONFIG_KALLSYMS
>  /* Lookup the address for a symbol. Returns 0 if not found. */
>  unsigned long kallsyms_lookup_name(const char *name);
> +int ftrace_lookup_symbols(const char **sorted_syms, size_t cnt, unsigned long *addrs);
>  
>  extern int kallsyms_lookup_size_offset(unsigned long addr,
>  				  unsigned long *symbolsize,
> @@ -103,6 +104,11 @@ static inline unsigned long kallsyms_lookup_name(const char *name)
>  	return 0;
>  }
>  
> +static inline int ftrace_lookup_symbols(const char **sorted_syms, size_t cnt, unsigned long *addrs);
> +{
> +	return -ERANGE;
> +}
> +
>  static inline int kallsyms_lookup_size_offset(unsigned long addr,
>  					      unsigned long *symbolsize,
>  					      unsigned long *offset)
> diff --git a/kernel/kallsyms.c b/kernel/kallsyms.c
> index 79f2eb617a62..1e7136a765a9 100644
> --- a/kernel/kallsyms.c
> +++ b/kernel/kallsyms.c
> @@ -29,6 +29,7 @@
>  #include <linux/compiler.h>
>  #include <linux/module.h>
>  #include <linux/kernel.h>
> +#include <linux/bsearch.h>
>  
>  /*
>   * These will be re-linked against their real values
> @@ -228,7 +229,7 @@ unsigned long kallsyms_lookup_name(const char *name)
>  	return module_kallsyms_lookup_name(name);
>  }
>  
> -#ifdef CONFIG_LIVEPATCH
> +#if defined(CONFIG_LIVEPATCH) || defined(CONFIG_FPROBE)
>  /*
>   * Iterate over all symbols in vmlinux.  For symbols from modules use
>   * module_kallsyms_on_each_symbol instead.
> @@ -572,6 +573,73 @@ int sprint_backtrace_build_id(char *buffer, unsigned long address)
>  	return __sprint_symbol(buffer, address, -1, 1, 1);
>  }
>  
> +#ifdef CONFIG_FPROBE
> +static int symbols_cmp(const void *a, const void *b)
> +{
> +	const char **str_a = (const char **) a;
> +	const char **str_b = (const char **) b;
> +
> +	return strcmp(*str_a, *str_b);
> +}
> +
> +struct kallsyms_data {
> +	unsigned long *addrs;
> +	const char **syms;
> +	size_t cnt;
> +	size_t found;
> +};
> +
> +static int kallsyms_callback(void *data, const char *name,
> +			     struct module *mod, unsigned long addr)
> +{
> +	struct kallsyms_data *args = data;
> +
> +	if (!bsearch(&name, args->syms, args->cnt, sizeof(*args->syms), symbols_cmp))
> +		return 0;
> +
> +	addr = ftrace_location(addr);
> +	if (!addr)
> +		return 0;
> +
> +	args->addrs[args->found++] = addr;
> +	return args->found == args->cnt ? 1 : 0;
> +}
> +
> +/**
> + * ftrace_lookup_symbols - Lookup addresses for array of symbols
> + *
> + * @sorted_syms: array of symbols pointers symbols to resolve,
> + * must be alphabetically sorted
> + * @cnt: number of symbols/addresses in @syms/@addrs arrays
> + * @addrs: array for storing resulting addresses
> + *
> + * This function looks up addresses for array of symbols provided in
> + * @syms array (must be alphabetically sorted) and stores them in
> + * @addrs array, which needs to be big enough to store at least @cnt
> + * addresses.
> + *
> + * This function returns 0 if all provided symbols are found,
> + * -ESRCH otherwise.
> + */
> +int ftrace_lookup_symbols(const char **sorted_syms, size_t cnt, unsigned long *addrs)
> +{
> +	struct kallsyms_data args;
> +
> +	args.addrs = addrs;
> +	args.syms = sorted_syms;
> +	args.cnt = cnt;
> +	args.found = 0;
> +	kallsyms_on_each_symbol(kallsyms_callback, &args);
> +
> +	return args.found == args.cnt ? 0 : -ESRCH;
> +}
> +#else
> +int ftrace_lookup_symbols(const char **sorted_syms, size_t cnt, unsigned long *addrs)
> +{
> +	return -ERANGE;
> +}
> +#endif /* CONFIG_FPROBE */
> +
>  /* To avoid using get_symbol_offset for every symbol, we carry prefix along. */
>  struct kallsym_iter {
>  	loff_t pos;

-- 
Masami Hiramatsu <[email-masked]>